One of conventional methods used for display apparatuses such as head-mounted displays (HMDs) is a method of directly drawing images on the retinas of the user's eyes by two-dimensionally scanning laser beams (hereinafter described as a laser scanning method) (for example, see Patent Reference 1). The display apparatus according to the laser scanning method is also known as a retinal scanning display, retinal irradiation display, retinal direct-drawing display, laser scanning display (RSD), direct-view-type display, virtual retinal display (VRD), and so on.
HMDs according to the laser scanning method generally include a scan unit which two-dimensionally scans laser beams emitted from a laser light source, and a deflection mirror, such as a lens or a mirror provided in front of the user's eyes, which deflects the beams toward the user's pupils so that the beams pass through the pupils and draw images on the retinas. Here, a point, near the user's pupils, at which the beams deflected by the deflection mirror are focused is hereinafter described as a “deflection focal point”. The terms “focal point” and “focal position” are also described as having the same meaning as the deflection focal point.
A rotation of the user's eyeballs without a movement of the head, as in the case of the user looking sideways, causes a change in the positional relationship between the user's pupils and the HMD mounted on the head. When this causes misalignment between a deflection focal point and the pupil position, the beams deflected by the deflection mirrors do not pass through the pupils, resulting in a situation where images cannot be drawn on the user's retinas. (Hereinafter, this situation is referred to as “pupil misalignment” or “pupil misalignment caused by eyeball rotation”.) The occurrence of the pupil misalignment depends on whether or not the beams, the diameter of which is usually 2 to 3 millimeters approximately, can pass through the user's pupils.
FIGS. 23 and 24 are explanatory diagrams of a pupil misalignment. When the deflection focal point is on the user's pupil as shown in FIG. 23, the beams deflected by the deflection mirror can pass through the user's pupil, which means that the pupil misalignment does not occur and images can be drawn on the retina. However, when the user's eyeball rotates to the left as shown in FIG. 24, the deflection focal point falls outside the pupil, thereby causing the pupil misalignment and thus images cannot be drawn on the retina.
In the case where the diameter of the user's pupil is 3 millimeters, for example, the pupil misalignment occurs when the pupil moves 1.5 millimeters or more, thereby hindering the user from viewing images. A trial calculation of the screen's view angle in the case of the pupil moving 1.5 millimeters shows that the display screen's view angle is approximately 14 degrees on each side (approximately 27 degrees in total for the left and right sides, and the same applies to the upper and lower sides) in the case where the distance between the user's pupil and the deflection mirror is 15 millimeters and the distance between the pupil and the rotation center of the user's eyeball is 10.5 millimeters. The pupil misalignment occurs when the user's line of sight moves outside such a range.
It is to be noted that FIGS. 23 and 24 are simplified diagrams illustrating the impact on beam refraction in a simplified manner. The impact on the beam refraction is illustrated in a simplified manner because, although the beams entering the user's eye are, in reality, refracted with the impact of the cornea, crystalline lens, and so on, what is important in this description is whether or not the beams can pass through the pupil.
As a countermeasure against the pupil misalignment, there is a method of providing the deflection mirror with plural deflection focal points (see Patent Reference 2, for example). As shown in FIG. 25, the deflection mirror having two deflection focal points prevents the pupil misalignment in some cases because even when the user's pupil moves to the left, a different deflection focal point falls on the pupil.
Further, an image display apparatus used in a head-mounted display and so on is an image display apparatus that is one form of personal, mobile display terminal apparatuses. To be wearable, such an image display apparatus generally has an eyeglass or monocular structure. With such an eyeglass-type head-mounted display, it is often the case that the user visually recognizes an output image of the image display apparatus and, at the same time, a background image which is seen through a portion of the image display apparatus corresponding to an eyeglass lens. In such a case of viewing the output image of the image display apparatus and the background image at the same time in an integrated manner, a solution of the problems below is desired.
To allow the user to view the output image, or a virtual image, and the background image in an integrated manner while preserving the function as eyeglasses, it is necessary for the image display apparatus to have an optical system that includes one or more half mirrors, and a lens or a concave mirror for forming virtual images. Consequently, there is a problem that the size and weight of such an image display apparatus makes it difficult for the user to use the apparatus for a long time. In addition, to pursue high-resolution output images, the image display apparatus becomes larger, resulting in a heavier burden on the user. Thus, in order to solve such problems, the image display apparatus used in a head-mounted display and so on is desired to be small and light-weight while being capable of displaying high-resolution images.
As an eyeglass-type image display apparatus as above, a retinal scanning or laser scanning image display apparatus has been proposed which uses a small, light-weight, and low-power-consuming laser diode array for the light source and which includes a Lippmann-Bragg volumetric hologram sheet capable of providing the optical system with multiple optical functions (see Patent Reference 3, for example). The left and right temples of the eyeglasses are provided with a small optical system such as a light source and a galvanometer mirror, a small driving circuit, and so on, to achieve miniaturization and a lighter weight of the image display apparatus.
When an output image is to be viewed using such an eyeglass-type image display apparatus, a situation sometimes arises in which a ray of light projected onto the user's pupil is shielded by the iris and thus the visual field is narrowed, hindering the user from viewing the entire or a part of the output image or causing uneven luminance in the image.
In view of this, a high-luminance white LED is used as a point light source, and a scattering plate is provided in the optical system, so that a light bundle from a spatial light modulator of the image display apparatus spreads widely, thereby broadening the width of the light bundle at a location in the vicinity of the user's pupil. An image display apparatus has been proposed which is, as a result of the above, capable of reliably guiding the light bundle from the spatial light modulator into the user's pupil even when the eyeball slightly shifts from a predetermined position, thereby preventing a narrow visual field and the unevenness in luminance (see Patent Reference 4, for example).
The occurrence of the pupil misalignment depends on whether or not the beams, the diameter of which is usually 2 to 3 millimeters approximately, can pass through the user's pupils. Therefore, the larger the angle of view (viewing angle) of images displayed by an HMD is, the larger the rotation angle of eyeball rotation is like to be, and the more likely it is that the pupil misalignment occurs. On the other hand, when the screen on which the HMD displays images is small and thus the screen's view angle is small, the rotation angle of an eyeball rotation for viewing an edge of the screen is relatively small, which means that the pupil misalignment is less likely to occur.
FIG. 26 is an explanatory diagram of a pupil misalignment. When the user's pupil is at a pupil position A, the pupil misalignment does not occur as long as the beams deflected by the deflection mirror (deflection unit) pass through a focal point A. In this case, the user can view images. However, when an eyeball rotation causes the pupil to move to a pupil position B, the pupil misalignment occurs because the beams focused on the focal point A cannot pass through the pupil. In this case, the user cannot view images.
As a countermeasure against the pupil misalignment, there is a method of providing the deflection unit with plural focal points (see Patent Reference 2, for example). In FIG. 26, in the case where the deflection mirror has two focal points A and B, the beams focused on the focal point A when the user's pupil is at the pupil position A, and the beams focused on the focal point B when the user's pupil is at the pupil position B both reach the retina. As a result, the pupil misalignment does not occur.
Moreover, to detect the user's line of sight, there is a method of using reflection of infrared light projected onto the user's eyes (for example, see Patent Reference 5), and a method of using reflection of a laser beam scanned on the user's eyes (for example, see Patent Reference 6).    Patent Reference 1: Japanese Patent No. 2932636    Patent Reference 2: U.S. Pat. No. 6,043,799    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 10-301055    Patent Reference 4: Japanese Unexamined Patent Application Publication No. 2000-249971    Patent Reference 5: Japanese Patent No. 2995876    Patent Reference 6: Japanese Patent No. 3425818